This clears out the combustion exhaust gases from the previous cycle and allows refilling with a clean mix of air and fuel.
As a turbocharger has some lag time coming up to speed, turbo-charged two-stroke diesels often display four-stroking when starting, or when suddenly accelerating from idle.
Some large engines, such as those from EMD, minimize this by using a turbocharger with an auxiliary mechanical drive to give better scavenging at low rpm.
Owing to the scaling laws of such small engines, this four-stroking is an unavoidable consequence of limitations on their scavenging at slow speeds.
However the same scaling laws also make the effects of four-stroking less severe and so the engines can idle satisfactorily in this mode without damage.
This can be nearly double the normal pressure, leading to excess noise and potentially failure of overloaded bearings in the connecting rod.
This may be done simply by cutting the ignition spark to one, thus increasing the load on the other cylinder and thus the power and gas-flow required.
[5] This has the drawback of wasting fuel in the un-ignited cylinder, potentially also risking oiling-up its spark plug and clogging the exhaust system.
[5] This avoids the risk of oil-fouling and routes all gas-flow through the operating cylinder, greatly increasing fuel economy.